I bought a PC as part of a larger system for resale to a customer. Due to some internal company mix-ups, the PC was ordered with Windows 7 Pro, but the customer wants it to have Windows 10. If I buy a new, full version of Windows 10 Pro and install it on the PC, will I be okay to sell it on to my customer? Do I need to get the OEM version, or will the retail version suffice? I tried contacting MS support, but the tech was only interested in selling me the OS and failed to answer my question.

4. Transfer. The provisions of this section do not apply if you acquired the software as a consumer in Germany or in any of the countries listed on this site (aka.ms/transfer), in which case any transfer of the software to a third party, and the right to use it, must comply with applicable law.
a. Software preinstalled on device. If you acquired the software preinstalled on a device (and also if you upgraded from software preinstalled on a device), you may transfer the license to use the software directly to another user, only with the licensed device. The transfer must include the software and, if provided with the device, an authentic Windows label including the product key. Before any permitted transfer, the other party must agree that this agreement applies to the transfer and use of the software.
b. Stand-alone software. If you acquired the software as stand-alone software (and also if you upgraded from software you acquired as stand-alone software), you may transfer the software to another device that belongs to you. You may also transfer the software to a device owned by someone else if (i) you are the first licensed user of the software and (ii) the new user agrees to the terms of this agreement. You may use the backup copy we allow you to make or the media that the software came on to transfer the software. Every time you transfer the software to a new device, you must remove the software from the prior device. You may not transfer the software to share licenses between devices.

Edit: Forgot to say yes, retail would be the correct licensing option here.

4. Transfer. The provisions of this section do not apply if you acquired the software as a consumer in Germany or in any of the countries listed on this site (aka.ms/transfer), in which case any transfer of the software to a third party, and the right to use it, must comply with applicable law.
a. Software preinstalled on device. If you acquired the software preinstalled on a device (and also if you upgraded from software preinstalled on a device), you may transfer the license to use the software directly to another user, only with the licensed device. The transfer must include the software and, if provided with the device, an authentic Windows label including the product key. Before any permitted transfer, the other party must agree that this agreement applies to the transfer and use of the software.
b. Stand-alone software. If you acquired the software as stand-alone software (and also if you upgraded from software you acquired as stand-alone software), you may transfer the software to another device that belongs to you. You may also transfer the software to a device owned by someone else if (i) you are the first licensed user of the software and (ii) the new user agrees to the terms of this agreement. You may use the backup copy we allow you to make or the media that the software came on to transfer the software. Every time you transfer the software to a new device, you must remove the software from the prior device. You may not transfer the software to share licenses between devices.

Edit: Forgot to say yes, retail would be the correct licensing option here.

4. Transfer. The provisions of this section do not apply if you acquired the software as a consumer in Germany or in any of the countries listed on this site (aka.ms/transfer), in which case any transfer of the software to a third party, and the right to use it, must comply with applicable law.
a. Software preinstalled on device. If you acquired the software preinstalled on a device (and also if you upgraded from software preinstalled on a device), you may transfer the license to use the software directly to another user, only with the licensed device. The transfer must include the software and, if provided with the device, an authentic Windows label including the product key. Before any permitted transfer, the other party must agree that this agreement applies to the transfer and use of the software.
b. Stand-alone software. If you acquired the software as stand-alone software (and also if you upgraded from software you acquired as stand-alone software), you may transfer the software to another device that belongs to you. You may also transfer the software to a device owned by someone else if (i) you are the first licensed user of the software and (ii) the new user agrees to the terms of this agreement. You may use the backup copy we allow you to make or the media that the software came on to transfer the software. Every time you transfer the software to a new device, you must remove the software from the prior device. You may not transfer the software to share licenses between devices.

4. Transfer. The provisions of this section do not apply if you acquired the software as a consumer in Germany or in any of the countries listed on this site (aka.ms/transfer), in which case any transfer of the software to a third party, and the right to use it, must comply with applicable law.
a. Software preinstalled on device. If you acquired the software preinstalled on a device (and also if you upgraded from software preinstalled on a device), you may transfer the license to use the software directly to another user, only with the licensed device. The transfer must include the software and, if provided with the device, an authentic Windows label including the product key. Before any permitted transfer, the other party must agree that this agreement applies to the transfer and use of the software.
b. Stand-alone software. If you acquired the software as stand-alone software (and also if you upgraded from software you acquired as stand-alone software), you may transfer the software to another device that belongs to you. You may also transfer the software to a device owned by someone else if (i) you are the first licensed user of the software and (ii) the new user agrees to the terms of this agreement. You may use the backup copy we allow you to make or the media that the software came on to transfer the software. Every time you transfer the software to a new device, you must remove the software from the prior device. You may not transfer the software to share licenses between devices.